The present invention relates to a method for producing a semiconductor device, and in particular, to a method for crystallizing a layer of semiconductive material on an insulating layer.
Since silicon is the most commonly employed material for use in crystallization/recrystallization processes, the term "silicon" will be used herein to describe processes wherein other semiconductive materials such as germanium, gallium arsenide and the like can be employed.
The terms "crystallizing" and "crystallization" are used herein to refer to both the process of heating an amorphous material to cause it to crystallize and the process of heating a polycrystalline material to cause it to recrystallize, since in both processes the material is crystallized from a molten state.
Silicon films on insulators can be used for producing large area device arrays suitable for flat-panel displays, dielectrically isolated devices for high-voltage and high-frequency integrated circuits, and other applications.
Silicon films have been produced by depositing on a suitable substrate amorphous or polycrystalline silicon in the form of a large area film or an array of islands. The silicon film is often formed as an island array when the resultant semiconductor devices are to be similarly spaced across the surface of the substrate. The entire silicon film or a portion thereof is subsequently heated by irradiation with a laser beam or other high-intensity light source to produce crystallization. Alternatively, the crystallization energy source has been an electron beam or a movable strip heater.
Silicon films have been formed on glass and fused silica substrates. By using glass substrates which approximate the thermal expansion of a deposited silicon film, thermal stress cracking during laser crystallization can be reduced or eliminated. Prior to depositing the silicon film on a glass substrate there is deposited thereon a barrier layer of refractory material. An ion barrier layer such as Si.sub.3 N.sub.4 and/or an electron barrier layer such as SiO.sub.2 can be employed. Such barrier layers are conventionally deposited by chemical vapor deposition (CVD) techniques. Various types of glass substrates are discussed in the publication by R. A. Lemons et al. entitled "Laser Crystallization of Si Films on Glass", Applied Physics Letters, Vol. 40, No. 6, 15 March 1982, Pages 469-471. This publication also indicates that the crazing of the silicon film due to a mismatch between fused quartz and silicon can be avoided by patterning the silicon films with narrow moats and small islands.
To prevent the silicon layer from agglomerating when it is melted, a capping layer is deposited on the surface thereof. As taught in U.S. Pat. No. 4,371,421, the capping layer may comprise a layer of Si0.sub.2, for example, formed by a method such as CVD or thermal oxidation over the first applied amorphous or polycrystalline silicon film. That patent further teaches that an additional layer of silicon nitride (Si.sub.3 N.sub.4) can be formed by a technique such as CVD over or under the silicon dioxide capping layer. The added Si.sub.3 N.sub.4 layer appears to further enhance the wetting properties of the encapsulation layer as compared with a single layer of SiO.sub.2. The thickness of the SiO.sub.2 capping layer is usually between 0.5 and 2.0 .mu.m. The Si.sub.3 N.sub.4 layer may have a thickness of a few hundred Angstroms.
Large area silicon films which do not have capping layers can be crystallized without agglomeration thereof if process parameters such as laser power, scan rate, etc. are very carefully controlled. But such a method is not conducive to a commercial operation because of the tight process tolerances. In general, a capping layer is required for surface smoothness and integrity, especially when the silicon film is in the form of islands. Furthermore, thin silicon films tend to agglomerate more readily than thicker ones.
The publication by L. Pfeiffer et al. entitled "Si-On-Insulator Films of High Crystal Perfection by Zone Melting Under a SiO.sub.2 Cap Provided with Vent Openings", Applied Physics Letters, Vol. 47, No. 2, 15 July 1985, pages 157-159, reports a marked improvement in the crystal perfection of zone melted thick (at least 15 .mu.m) Si-on-insulator films that were prepared for melt processing by etching an array of openings in the SiO.sub.2 capping layer. That publication theorizes that the improvement is due to the creation of venting paths that reduce the level of excess dissolved Si0.sub.2 in the molten silicon before crystallization. The discrete vent openings of the Pfeiffer publication may cause the following disadvantages. If the vent dimensions are too large, silicon agglomeration may occur within a vent. The vented areas and the non-vented areas of the silicon film possess different optical properties. Furthermore, an extra photolithographic step is required to form the vents, thereby increasing the processing time and cost.